Screen printing is a well-established art in which ink is transferred through openings in a screen defining a desired pattern onto a substrate. This process is used to print on a variety of substrates, such as tee shirts, textile, plexiglass sheets and printed circuit board substrates. After a stencil has been made defining the desired pattern, it is mounted in a frame, the frame is positioned over the substrate, ink is applied to the upper surface of the screen and the ink is then transferred through the screen openings onto the substrate.
Conventional methods for transferring ink include use of squeegees or vacuum pressure. In the former method a squeegee slightly wider than the image is placed behind the ink, pressed down on top of the screen and moved with even pressure across the image area forcing the ink through the open spaces on the screen onto the substrate. When the screen is raised the ink passed through the holes remains on the substrate. In the latter method ink is placed over the openings and a vacuum is drawn in the region between the substrate and screen, causing the screen to deflect downwardly into contact with the substrate, whereupon the ink is drawn through the openings into contact with the substrate.
A number of problems exist in prior art vacuum printers. First, it is difficult to precisely align the screen with the underlying substrate, especially in applications having small tolerances, such as the manufacture of printed circuit boards.
Second, the substrate/ink interface forms a meniscus coupling at the time of printing that bonds the print screen and substrate together. This coupling interferes with release of the screen from the substrate which can smear the ink pattern deposited on the substrate. This problem may be especially acute if the print screen is larger than the substrate.
Accordingly, one object of the present invention is to provide a new and improved apparatus for and method of vacuum screen printing.
Another object of the invention is to provide a new and improved apparatus for and method of manufacturing printed circuit boards.
Yet another object of the present invention is to provide a screen printing apparatus in which the printing screen may be accurately registered with the underlying substrate.
A further object of the invention is to provide a vacuum screen printing system which minimizes smearing of the printed pattern on the substrate.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.